1. Field of the Invention
The present invention relates to the field of measurement of a brain function disclosed in, for example, Japanese Patent Laid-Open No. 2000-237194.
2. Description of the Related Art
An example of measurement of a brain function will be described in conjunction with FIG. 1A and FIG. 1B. FIG. 1A shows the configuration of a brain function measurement system and the relationship thereof to a subject. FIG. 1B shows an example of the distribution of positions S at which a light irradiating means for irradiating the subject's head with light is located, and positions D at which a receiving optical fiber for receiving light that has been applied to the subject's head and transmitted thereby is located.
The brain function measurement system includes: a plurality of light sources 102a to 102d that generates light waves having difference wavelengths (the light sources 102a and 102c generate light having a wavelength of, for example, 780 nm, and the light sources 102b and 102d generate light having a wavelength of, for example, 830 nm); oscillators 101a and 101b and oscillators 101c and 101d that modulate at different frequencies the intensities of light waves emitted from the plurality of light sources 102a and 102b or light sources 102c and 102d; a plurality of light irradiating means for irradiating, which a coupler 104a produces using the intensity-modulated light waves having propagated along optical fibers 103a and 103b, and light, which a coupler 104b produces using the intensity-modulated light waves having propagated along optical fibers 103c and 103d, different positions on the head skin of a subject 106 with light over light irradiating optical fibers 105a and 105b; and a plurality of pieces of light receiving means composed of a plurality of receiving optical fibers 107a to 107f, which is disposed so that the ends thereof will be located equidistantly from (for example, 30 mm away from) the light-applied positions, that is, the plurality of pieces of light irradiating means, and light receivers 108a to 108f disposed at the other ends of the optical fibers 107a to 107f. 
In the example of FIG. 1A, the three receiving optical fibers (D in the drawing) 107a to 107c and three receiving optical fibers 107d to 107f are, as shown in FIG. 1B, disposed around the light irradiating optical fibers (S in the drawing) 105a and 105b, so that light waves transmitted by a living body will be converged on the optical fibers and detected. The detected light waves transmitted by the living body are photoelectrically converted by the light receivers 108a to 108f. The light receiving means detects light, which is transmitted by the subject's intracranial regions while being reflected therefrom, and converts the light into an electrical signal. The light receivers 108a to 108f are realized with photoelectric conversion elements such as photoelectric multipliers or photodiodes.
Electrical signals that represent the intensities of light waves transmitted by a living body and that result from photoelectric conversion performed by the light receivers 108a to 108f (hereinafter, living body-transmitted light intensity signals) are transferred to lock-in amplifiers 109a to 109h. The light receivers 108c and 108d detect the intensities of living body-transmitted light waves converged on the receiving optical fibers 107c and 107d that are located equidistantly from the light irradiating optical fibers 105a and 105b respectively. The signals proportional to the light intensities detected by the light receivers 108c and 108d are each separated into two portions and transferred to the lock-in amplifiers 109c and 109e or the lock-in amplifiers 109d and 109f. Signals that are the outputs of the oscillators 101a and 10b as well as 101c and 101d modulated in intensity at intensity modulation frequencies are transferred as signals of reference frequencies to the lock-in amplifiers 109a to 109d, and 109e to 109h respectively. Consequently, the living body-transmitted light intensity signals representing the intensities of light waves emitted from the light sources 102a and 102b are separated from each other and transmitted from the lock-in amplifiers 109a to 109d. The living body-transmitted light intensity signals representing the intensities of light waves emitted from the light sources 102c and 102d are separated from each other and transmitted from the lock-in amplifiers 109e to 109h. 
The transmitted light intensity signals separated from one another in units of a wavelength and transmitted from the lock-in amplifiers 109a to 109h are analog-to-digital converted by an analog-to-digital converter (hereinafter, an A/D converter) 110, and then transferred to a measurement control computer 111. The measurement control computer 111 uses each of the transmitted-light intensity signals, that is, detection signals produced at detected positions to thus arithmetically or logically calculate relative changes in an oxyhemoglobin concentration, a deoxy-hemoglobin concentration, and a total hemoglobin concentration. The relative changes are stored as time-sequential information on each of the measured positions in a storage device included in the computer 111. Herein, the change in the total hemoglobin concentration is calculated as the sum of the changes in the oxyhemoglobin concentration and deoxy-hemoglobin concentration.
On the other hand, in order to measure a brain function of a subject, a predetermined stimulus or task is applied to the subject and the subject's response to the stimulus or task is assessed. A centralized control/data processing/result display computer 114 issues a command to the measurement control computer 111. The measurement control computer 111 in turn uses a stimulus/task command presentation device 113 to apply a stimulus/task instruction to the subject according to a prepared stimulus/task instruction sequence. A response to the stimulus/task instruction made by the subject's brain is optically measured as described above. The centralized control/data processing/results display computer 114 and the measurement control computer 111 communicate required information to each other.
Conventionally, in order to assess a subject's response to a stimulus or task, the significance of a signal representing an average response obtained as a result of repetitive measurements is tested based on the amplitude of the signal. A significantly active area is then identified (refer to Japanese Unexamined Patent Application Publication No. 2000-237194).